During a starting process of a synchronous motor, an electromagnet of a stator generates an alternating magnetic field, dragging a permanent magnet rotor to oscillate. If the rotor obtains enough kinetic energy, oscillation amplitude of the rotor will keep increasing, and finally a rotational speed of the rotor will be rapidly accelerated to be synchronize with the alternating magnetic field of the stator.
In practice, when the motor starts from a rest position, a current in a stator winding does not change suddenly due to physical characteristics of the winding, so the current in the stator winding rams up slowly. An input power Pinput to the motor also rams up slowly, as the input power Pinput can be expressed in terms of as Pinput=VBemf×Imotor, wherein VBemf is the back electromotive force, and I is current in the stator winding. If the input power Pinput is not large enough to overcome starting friction between a shaft and a shaft sleeve of the motor, and inertia of a motor load such as a pump or a fan, the motor will keep a resting state and will not start normally even if powered on.